The North Sulawesi Subduction Zone is one of the youngest active subduction systems in the western Pacific. In western Sulawesi, the Palu–Koro strike–slip fault connects with the westward-extending North Sulawesi Trench, forming a distinctive subduction–transform fault system. Understanding the crustal structure beneath the Celebes Sea and the geometry of the Palu–Koro fault is crucial for assessing regional deformation, rupture dynamics, and seismic hazards. In this study, we analyse data from nine ocean bottom seismometers (OBSs) deployed across the Palu–Koro fault using the receiver function H–κ stacking method to estimate crustal thickness. Our results reveal a shallow Moho (~8 km depth) beneath the Celebes Sea, in contrast to significantly greater depths (~25 km) beneath eastern Kalimantan and northern Sulawesi. Sharp variations in Moho depth near the Palu–Koro fault suggest the juxtaposition of two distinct crustal blocks. Combining S-wave velocity structures and local seismicity catalogue, we infer that the Palu–Koro fault is a left-lateral, supracrustal strike–slip fault extending into the Celebes Sea. These findings provide new geophysical constraints on the interplay between strike–slip faulting and subduction retreat, with implications for the generation of tsunamis by submarine earthquakes in this tectonically complex region.